(1) Field of the Invention
The present invention relates to a cell safety valve in which a thin valve plate is formed in an opening hole in a sealing plate for sealing a cell such that if an internal cell pressure exceeds a predetermined value this valve plate may break so as to release a gas in the cell out of the cell, and a method for manufacturing the same.
(2) Description of the Related Art
Recently, besides LiCoO2 and other lithium-containing composite oxides used as the positive electrode material, non-aqueous electrolyte cells using as the negative electrode material, such materials as lithium-aluminum alloy and carbon materials, which are capable of intercalating and deintercalating lithium ions, are attracting public attention as being capable of improving capacity.
When such a non-aqueous electrolyte cell is mishandled, e.g., put in fire, recharged, or discharged under abnormal conditions, a great amount of gas may be produced in the cell. Unless the gas in the cell is released out quickly, it may burst or ignite problematically. To prevent such a problem, such a cell is provided with a safety valve for releasing the gas in the cell out of it quickly at the time of abnormality. As such a safety valve, the following valves have been proposed:
(1) a valve that, as described in Japanese Unexamined Patent Application No. 11-250885 (see FIGS. 1 through 4), in an opening hole 21a in a ring-shaped base material 21, a cladding material 22 (with a thickness of about 10% of that of the base material) formed by two sheets of aluminum-based materials and constituting a valve plate is welded or pressure-welded to form a safety valve 23, which is in turn mounted to a sealing plate 24 (one that is a so-called cladding-material-spec safety valve);
(2) a valve that, as described in Japanese Unexamined Patent Application No. 11-250885 (see FIGS. 5 and 6), a break groove 26 is formed at around the middle of an opening hole 25a in a sealing plate 25; and
(3) a valve that, as described in Japanese Unexamined Patent Application No. 11-273640 (see FIGS. 7 and 8), a dome-shaped thin valve plate 29 is formed starting from the lower end of an opening hole 28a in a sealing plate 28.
Those conventional safety valves, however, have had the following problems.
Problems of Type (1) Safety Valve
This safety valve 23 may have irregularities in strength of welding or pressure welding of the base material 21 and the cladding material 22, which may in turn damage the cladding material 22 when the safety valve 23 is mounted to the sealing plate 24, thus causing leakage of an electrolyte or increasing the cell-to-cell difference in the operating pressure of the safety valve.
Problems of Type (2) Safety Valve
This safety valve, although it reduces the cell-to-cell difference in the safety-valve""s operating pressure, may have irregularities in the open area of a valve plate 27 upon the breaking of the safety valve, so that in case the open area is small, amount of gas production may be more than the amount of gas release. This may prevent the safety valve from having its own functions sufficiently, thus causing the cell to ignite or burst problematically.
Problems of Type (3) Safety Valve
This safety valve, although it can release a lot of gas produced quickly due to an enlarged open area of the valve plate 29, has the thin valve plate 29 formed from the lower end of the opening hole 28a, so that the valve plate 29 may be damaged by a jig etc. to produce cracks etc. if it had vibration or shock on it during assembly of the cells, thus causing the leakage of the electrolyte.
It is an object of the present invention to provide such a cell safety valve and a method for manufacturing the same that can ensure a sufficient open area during the operation of the safety valve while reducing the cell-to-cell difference in the operating pressure of the safety valve.
It is another object of the present invention to provide such a safety valve and a method for manufacturing the same that can prevent an electrolyte from leaking.
To achieve the above-mentioned objects, a cell safety valve according to a first aspect of the invention has its thin valve plate formed on a sheet-shaped sealing plate for sealing the cells such that if the internal cell pressure exceeds a predetermined value, the valve plate may break to release the gas in the cell to the outside, wherein the valve plate has a dome-shaped dome portion formed thereon and also at its middle or near it has a break groove for facilitating the breakage thereof.
Since thus the valve plate has the break groove at its middle or near it for facilitating the breakage thereof, if the internal cell pressure rises abnormally, the valve plate breaks surely starting from the break groove; in addition, as the valve plate has the dome-shaped dome portion formed thereon, after the valve plate has thus started breaking starting from the break groove, the peripheries of the dome portion also break by increased stress due to the gas. Therefore, even with some irregularities in the thickness of the valve plate, the cell-to-cell difference can be reduced in the operating pressure of the safety valve.
A second aspect of the invention is characterized in that the cell safety valve according to the first aspect of the invention, wherein the above-mentioned dome portion is provided one.
A third aspect of the invention is characterized in that the cell safety valve according to the second aspect of the invention, wherein the above-mentioned break groove is formed in the periphery of the above-mentioned dome portion.
The break groove is thus formed in the periphery of the dome portion, to further facilitate the breaking of the valve plate, thus reducing the cell-to-cell difference in the operating pressure of the safety valve.
A forth aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein the above-mentioned dome portion is provided two or more.
The dome portion is thus provided two or more, to ensure a sufficient open area during the operation of the safety valve.
A fifth aspect of the invention is characterized in that the safety valve according to the forth aspect of the invention, wherein the above-mentioned break groove is formed in the periphery of at least one of the above-mentioned two dome portions or more.
Such a configuration gives almost the same actions and effects as those of the third aspect of the invention.
A sixth aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein the above-mentioned valve plate as a whole is disposed between an imaginary plane flush with the outside surface of the above-mentioned sealing plate and an imaginary plane flush with the inside surface of the above-mentioned sealing plate.
In such a configuration, the valve plate does not come in direct contact with the jig etc., so that even in case of a vibration or shock during the assembly of the cells, it does not cause the jig etc. to damage the valve plate, thus inhibiting the electrolyte from leaking.
A seventh aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein the above-mentioned dome portion bulges in a direction toward the outside of the cell so as to form a dome shape.
Such a configuration further ensures the operations of the safety valve.
An eighth aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein the thickness of the above-mentioned valve plate is regulated to 0.1 through 10% of that of the above-mentioned sealing plate.
The thickness of the valve plate is thus regulated because a valve plate thickness of less than 0.1% of the sealing plate thickness is so thin that may cause leakage of the electrolyte, while a valve plate thickness of more than 10% of the sealing plate is so thick that excessively increase the cell-to-cell difference in the operating pressure of the safety valve.
A ninth aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein the plane shape of the above-mentioned valve plate is a true circle, an ellipse, or a quadrangle.
Among a true circle, an ellipse, and a quadrangle exemplified as the plane shape of the valve plate, an ellipse or a quadrangle is desirable. This is because in contrast to a true-circular shape of the valve plate that causes uniform stress to be applied on its peripheries and makes it difficult to break which in turn increases the cell-to-cell difference in the operating pressure of the safety valve, while an elliptic or rectangular shape of the valve plate causes stronger stress to be applied on the longer side and surely causes the valve plate to break starting from the longer side, thus reducing the cell-to-cell difference in the operating pressure of the safety valve.
A tenth aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein the valve plate and the sealing plate are molded in one piece.
Such a configuration reduces the number of components of the safety valve and makes it possible to decrease the costs for manufacturing the cells.
An eleventh aspect of the invention is characterized in that the safety valve according to the first aspect of the invention, wherein besides the above-mentioned break groove, a break aiding groove is formed near the periphery of the above-mentioned valve plate.
In such a configuration, as the internal cell pressure rises, a displacement also increases near the breaks of the dome portion, so that particularly for cells with a small-sized valve plate (thin cells), even a small rise in the internal cell pressure ensures stable operations. Further, in the manufacturing of the valve plate, its tolerance can be relaxed, to facilitate quality control and metal-mold adjustment, thus improving the productivity.
To achieve the above-mentioned objects, a twelfth aspect of the invention comprises a valve plate forming step of forming a valve plate wherein a sheet-shaped sealing plate for sealing the cells is provided with a dome-shaped dome portion and a break groove is formed at the middle or near it for facilitating the breaking of the valve plate by plasticity working.
By such a method, the cell safety valve according to the first aspect of the invention can be formed when the components of the sealing plate are worked, thus improving the productivity.
A thirteenth aspect of the invention is characterized in that the method according to the twelfth aspect of the invention, wherein during the above-mentioned valve plate forming step, the valve plate is formed between an imaginary plane flush with the outside surface of the above-mentioned sealing plate and an imaginary plane flush with the inside surface of the above-mentioned sealing plate.
By such a method, the cell safety valve according to the sixth aspect of the invention can be prevented from being damaged when it comes in contact with jigs or any other sealing elements during manufacturing.
A fourteenth aspect of the invention is characterized in that the method according to the twelfth aspect of the invention, wherein during the above-mentioned valve plate forming step, the dome portion is formed in such a way as to bulge in a direction toward the outside of the cells.
By such a method, a further surely operating valve can be made in manufacturing of the cell safety valve according to the seventh aspect of the invention.
A fifteenth aspect of the invention is characterized in that the method according to the twelfth aspect of the invention, wherein during the above-mentioned valve plate forming step, besides the above-mentioned break groove, a break aiding groove is formed near the periphery of the above-mentioned valve plate.
By such a method, a more surely operating valve plate can be made in manufacturing the cell safety valve according to the eleventh aspect of the invention.
A sixteenth aspect of the invention is characterized in that the method according to the twelfth aspect of the invention, wherein after the above-mentioned valve plate forming step, there is included an annealing step of annealing the valve plate.
If a thin valve plate is formed by drawing, the material for the valve plate has higher hardness, so that with the increasing mechanical strength of the material itself, the cell-to-cell difference in the operating pressure of the safety valve may be larger. By annealing the valve plate after the valve plate forming step, however, the valve plate material has lower hardness, so that with the decreasing mechanical strength of the material itself, the cell-to-cell difference in the operating pressure of the safety valve is reduced.